The Late Eocene-Oligocene sediments of the Te Kuiti Group represent a classical transgressive sequence from basal nonmarine coal measures to overlying shallow marine mudstones, sandstones, and limestones. Paleoenvironmental conditions in the South Auckland region during the Paleogene were such that basement Mesozoic sandstones and mudstones, the only local source rocks for terrigenous material, exerted a relatively subordinate role on the mineralogy of the Te Kuiti Group sediments accumulating in adjacent depocentres. Instead, the sediments include abundant transformed, authigenic, and biogenic mineral associations reflecting conditions pertaining in each of the pedogenetic, paludal, and temperate latitude, shallow marine environments. A kaolinite-quartz-siderite mineralogy dominates the nonmarine beds, and a low-Mg calcite-smectite-quartz-sodic plagioclase-glauconite mineralogy characterises the bulk of the marine deposits. The mineralogy bears the imprint of a complex set of factors in which the influences of climate, soil provenance, depositional environment, and diagenesis greatly dominate over the influences of tectonics and source-rock provenance.